Filter



June 18, 1935. c. w. HANSELL FILTER Filed July 7, 1927 a 2 FEEE INVENTOR CLARENCE W. HANSELL BY 9 (l.

% /Qdouw A ORNEY Patented June 18, 1935 UNITED I STATES FILTER Clarence W. Hansell, Rocky Point, N. Y., assignor to Radio Corporation of America, a ,ccrporation of Delaware Application July 7, 1927, Serial No. 203,901

10 Claim. This invention relates to filters for discriminating between energies of different frequencies, and more particularly to filters for selecting desiredfrequencies from undesired frequencies when the difference is very small relative to the frequencies involved.

In employing high frequencycarrier waves for communication purposes there frequently arise situations where it is desirable to discriminate between frequencies, and filters have assumed great importance. For example, it is often desired after modulation to suppress a carrier and to utilize only one of the two side bands resulting from modulation of the carrier. When very high carrier frequencies are employed the percentage difference between .the two side bands is exceedingly small, and separation becomes diflicult. It is an object of my invention to provide a highly selective filter, and this-I do by employing a piezo-electric crystal having a resonance frequency equal to the desired frequency.

Owing to the fact that the crystal and its holder form a. condenser through the capacitance of which frequencies may be transferred which differ from the resonance frequency of the crystal it is desirable to so arrange the filter circuits that the capacitance of the crystal and crystal holder is neutralized, and to do this is a further object of my invention. The crystal is mounted in one arm of a symmetrical impedance bridge, and the input and output circuits of the filter are connected to different pairs of opposite corners of the bridge, thus obviating any coupling between the input and output circuits by reason of the capacitance of the crystal and its holder.

Because its resonance curve is exceedingly sharply defined a crystal filter may be too selective for many purposes. To fulfill the need for a band filter, rather than a single frequency filter, while retaining the advantages of a crystal filter, is a still further object of my invention.

The widening of the filter'characteristic maybe accomplished in a number of ways, and insome cases it proves preferable to use a combination of methods. 7 One method is to employ a plurality of crystals of different adjacent resonance frequencies distributed over the dwired frequency band and connected electrically in parallel. In this manner the frequency selection may be widened from a sharp peak to a well-defined band, the width of which will depend upon the range of frequency of the various crystals employed. Another expedient for broadening the filter characteristic is the use of a parallel resonant circuit shunted across the output circuit of the crystal. In such case, in effect, the crystal filter may be compared to a single section of a Camp- (Cl its-44) bell band filter, wherein the series resonant circuit has been replaced by the crystal. The crystal filter has the advantage of being so sharply selective that the necessity for more than asingle section probably will neverarise.

The invention is described more completely in the following specification, which is accompanied by a drawing in which Figure l. is a wiring diagram fora simple form ,of crystal filter;

Figure 2 is a modification of Figure 1 rearranged in the form of a bridge;

Figure 3 shows the use of a plurality of crys- I tals; Figure 4 is a wiring diagram for a modification employing a parallel resonant output circuit; 7

Figure 5 is explanatory of Figure 4;

Figure 6 is a resonance curve for the modification shown in Figure 4; and

Figure '1 is an arrangement which is a combination of Figures 3 and 4. v I

Referring to Figure l, a crystal 2 is placed between the electrodes of a crystal holder 4 which is series connected between an input coil 6 and an output coil 8. For neutralization, either the input or output coil may be divided and a neutralizing condenser Ill connected in circuit as shown in' Figure 1. The filter will, of course, work equally well in both directions, and therefore the. coil 8 may be used as an input circuit and the coil 6 as an output circuit. In the simplest case .the V not necessarily be inductively coupled, for in some cases capacitive coupling may work equally well, and Figure 2 is included to show the more general case of a reactance bridge comprising a crys-'- tal and holder 2, 4 in'one arm of a bridge, a condenser III in another arm, and reactances l2 and I4 constituting the remaining arms of the bridge. The circuits l6 and I8 may be the input and output circuits, or vice versa. The reactances I2 and Il may be capacitive or inductive, and equal or unequal, provided that the bridge is brought to symmetry by proper adjustment of the condenser Hi. The circuits [6 and I8 cannot interact with respect to energy transferred thru the capacitance of the crystal and its holder because of the symmetrical coupling of the input and out- I In Figure 3 the arrangement is much like that shown in Figure 1 with the exception that in this case a plurality-of crystals 2, 2', 2 are shown between the electrodes of the crystal holder 4. As is schematically indicated in the figure, the plurality of crystals may be situated between the single pair of electrodes of one crystal holder, or

there may equally well be employed a plurality of separate crystals and crystal holders electrically connected in parallel, as indicated in Figure 7.

In Figure 4 there is a crystal and holder 2, l, a

divided input coil 6, a neutralizing condenser l0,

and across the output circuit there is connected a parallel resonant circuit 20. The output circuit in this case is shown connected to the grid and cathode of an amplifying tube 22, but this is not material to the present invention and has been included only to show that the output circuit need not necessarily be coupled to a. next succeeding circuit by means of transformer coupling. The parallel resonant circuit 20 serves to broaden out the resonance curve of the filter.

current flow which tends to raise the potential at the point A. On the other hand, for a frequency slightly below resonance the series resonant circuit becomes predominantly capacitive, the parallel resonant circuit becomes predominantly inductive, again tending to create series resonance, and so raising the potential at the point A. In consequence a resonance curve such as 22; in Figure 6, is changed to a resonance curve like that numbered 25.

In Figure 7 there is suggested a crystal filter for the selection of a frequency band governed by the use of both a plurality of. parallel connected crystals and a parallel resonant output circuit. The combined circuit has certain advantages not found in the individual circuits, for the reason that by using the parallel resonant output circuit, as well as the plurality of crystals, the crest of the band filter curve may be smoothed out.

While one of the important advantages of the crystal filter is the greatselectivity-of only a single section, there may be used if necessary, a plurality of sections in concatenation. It is also clear that the circuit shown in Figure 1 may be connected across a circuit, instead of in series with a circuit, in order to eliminate an undesired frequency, the neutralization serving to prevent the elimination of other frequencies through the natural capacitance of the crystal and its holder.

I'claim:

1. A band pass filter comprising, a plurality of piezo-electric crystals electrically connected in parallel and having different adjacent resonance frequencies, and means for neutralizing the interelectrode capacity of the crystals.

2. In combination, an input circuit containing a plurality of frequencies, an output circuit. and

' bridge.

interposed between said input and output circuits a band pass filter comprising a pair of electrodes and a plurality of piezo electric crystals situated between said electrodes and having different, adjacent, resonant frequencies, each crys tal having a surface adjacent each electrode whereby a band of frequencies is passed to said output.

3. A crystal band filter comprising an input circuit including an input coupling reactance, an output circuit including an output coupling reactance, and means coupling said circuits including a plurality of parallel connected piezo-electric crystals of different resonance frequencies distributed over the desired frequency band for passing a band of frequencies from said input to said output.

4. A crystal band filter comprising an input circuit, an output circuit, means coupling said circuits including a plurality of parallel connected piezo-electric crystals having diiferentresonance frequencies distributed over the desired frequency band of frequencies are transmitted by the filter having different adjacent resonance frequencies for passing from said input to said output a' band of frequencies.

6. A crystal band filter comprising an input circuit, a parallel resonant output circuit, and means coupling said circuits including a plurality of parallel connected piezo-electric crystals hav ing different adjacent resonance frequencies for passing a band of frequencies from said input to said output.

7. A crystal band filter comprising an input circuit, a parallel resonant output circuit, means coupling said circuits including a plurality of parallel connected piezo-electric crystals having different adiacent resonance frequencies, electrodes for said crystals, and means to neutralize the effect of the capacitance of said crystals and electrodes.

8. In electrical apparatus, a bridge circuit having a condenser in each of three arms of the bridge and a piezo-electric crystal and its associated electrodes connected in the fourth arm of the bridge, and, input and output circuits across opposite corners of the bridge.

9. A band pass filter comprising a plurality of piezo-electric crystals electrically connected in parallel and having different resonance frequencies, each crystal passing currents of frequencies within a narrow portion of said band, the portionsof said band passed by the various crystals being adjacent and overlapping each other, and means for neutralizing the interelectrode capacity of said crystals.

10. A crystal filter comprising a symmetrical bridge. having four arms, a piezo electric crystal in one of said arms, and a capacitance in each of said other arms, at least one of said capacitances being variable for neutralizing the capacity on said crystal andfor enabling adjustment of said CLARENCE W. HANSELL. 

